There is disclosed a method of detecting the orientation of a face of a human being as an object in D-596 “real-time extraction of an image of a human being oriented in a specified direction” on p 7 to 308, Extended Abstract to National Spring Meeting of the Telecommunication Science Association, 1991. In this method, a human face is imaged with a monochromatic camera, and the resulting image data is binary-coded and a human being whose face is directed to the front side is retrieved based on the distance between the center of an area containing eyes, a nose and a mouth and the center of a head. There is also disclosed a method entitled: “application to a command input of a method for detecting a face orientation by an image seen with a sole eye” in a thesis journal D-11 vol. J72-D-II Mo. 9 pages 1441 to 1447, of the Telecommunication Science Association, September 1989. In this method, a human face is modelled as a triangle constituted by three characteristic points, with distances between respective three points being given as known data. From the projection between these three points, the positions of the point of gravity of the triangle and the direction of the normal line are detected as the face orientation in a three-dimensional space.
In the Japanese Laying-Open Patent H-7-23934, it is proposed to find the orientation of the face based on the ratio L/W between the width L of the left head hair area L and the width W of the face area and on the ratio L/R between the width L of the right head hair area and the width R of the right head hair area.
However, if, when the characteristic points of the human face are both eyes and the nose etc, the face is movable over a wide range, both eyes become undetectable when the face is turned beyond a certain threshold angle, as a result of which characteristic points of the human face become unmeasurable.
Moreover, it is difficult to detect the positions of both eyes and the mouth from the head automatically and robustly. Moreover, in the latter method, in which the limiting conditions that the distances of the three points are known from the outset, it becomes difficult to automate the processing.
In the method described in Japanese Laying-Open Patent H-7-23934, in which the image is acquired using an IR camera, it is not only impossible to guess the face orientation from the image photographed using an routine camera, but the cost is raised significantly.